David Cameron has told the MPs that fight by bombing on so-called Islamic State in Syria will “keep the British people safe”.
But Labour leader Jeremy Corbyn said the PM’s case “doesn’t stack up” as MPs debate whether to back air strikes.
Mr Cameron faced calls to apologise for saying opponents of military action were “terrorist sympathisers”, with Mr Corbyn saying it “demeaned” his office.
The PM declined to apologise, but said there was “honour” in voting for or against military action.
The 10 hour Commons debate will end with a vote on whether the UK joins others such as France, the US and Russia in bombing targets in Syria.